The present invention relates to a golf ball having a spherical surface. More particularly, it relates to a golf ball provided with dimples of a characteristic shape on the spherical surface and having excellent flying performance.
For the conventional golf ball, the design freedom for dimples is degraded by the restriction on the fabricating technique such as the structure of a die used for manufacturing the golf ball and the restriction on the performance such as the enhancement of flying performance of golf ball. Therefore, it is difficult to enhance the flying performance of golf ball while dimples having a characteristic shape such as to improve the aesthetic appearance of golf ball are adopted.
In recent years, the performance of golf gear such as a golf ball and a golf club has been enhanced. Therefore, the golf ball is often flied by being hit under a low spin condition, and a low spin condition of about 2000 rpm is not rare. When the golf ball is flied under such a low spin condition, in the region of ball trajectory from hitting to the highest terminal point, a drag, which is an air resistance against the flying golf ball, is small, so that the carry of golf ball increases. However, in a low-speed region after the highest terminal point of ball trajectory, a lift for floating the golf ball cannot be obtained sufficiently, so that the carry of golf ball decreases. Therefore, it has been demanded that the flying performance of golf ball be enhanced by devising the shape of dimples considering the balance between the drag and lift under such a low spin condition.
Next, the relationship between dimples and flying performance is explained. The flying performance can be enhanced by regulating the volume percentage of dimples (total volume of dimples/volume of an imaginary spherical body (imaginary spherical body: a golf ball assumed to have no dimples)), the area percentage of dimples (total area of dimples/surface area of an imaginary spherical surface (imaginary spherical surface: the spherical surface of a golf ball assumed to have no dimples)), or the like for golf balls having various structure. The flying performance can be enhanced by devising the cross-sectional shape of dimples (ratio of depth to diameter of dimple, smoothness, etc.). Further, in the case where a plurality of dimples are arranged on the whole spherical surface of golf ball so as to be symmetrical, high aerodynamic characteristics can be obtained, and therefore the flying performance can be enhanced.
Golf balls having variously shaped dimples are present. The literature relating to such golf balls is cited below. Japanese Unexamined Patent Application Publication No. 2002-534237 discloses a golf ball provided with dimples having a sickle-shaped contour. Japanese Unexamined Patent Application Publication Nos. 8-276035 and 10-108921 disclose a golf ball provided with connecting grooves having a straight line shaped contour, which are arranged so as to connect a plurality of dimples to each other and formed into a concave shape having a depth shallower than the depth of dimple.
However, for the golf ball described in Japanese Unexamined Patent Application Publication No. 2002-534237, the dimples are formed so that the contour of dimple is greatly curved into a sickle shape. In the case where such dimples are arranged at a plurality of locations on the spherical surface of the golf ball, sufficient flying performance is possibly not provided.
For the golf ball described in Japanese Unexamined Patent Application Publication Nos. 8-276035 and 10-108921, the contour of the connecting groove connecting the dimples to each other is of a straight line shape, and not of a streamline and smooth shape. Due to such connecting grooves, the drag at the time when the golf ball is flied may increase, and therefore sufficient flying performance is possibly not provided. The depth of the connecting groove is shallower than the depth of the dimple, and the depths of the dimple and the connecting groove are uneven. Therefore, it is intricate to regulate the volume of dimples by adjusting the dimensions of dimples, so that a golf ball having enhanced flying performance is difficult to manufacture.